This invention relates generally to a system and method for depositing a thin film of material onto a surface of an object, such as a semiconductor or thin film head substrate, and in particular to a dual collimation system and method for depositing thin material films onto a semiconductor or a thin film head substrate.
Plasma sputtering is a physical vapor deposition (PVD) technique for thin film deposition. The process of sputtering a thin film onto an object is well known. The sputtering process may be used to deposit a thin film of material onto a plurality of different surfaces which may include a magnetic media substrate, magneto-optical media substrate, a semiconductor substrate, a thin-film head substrate, a flat-panel display substrate and the like. In the sputtering process, it is desired to place a thin film of atoms of a particular type, such as a cobalt, onto the substrate. A typical sputtering apparatus may include a piece of target material, a vacuum or low-pressure sputtering chamber, and a substrate located on a substrate holder beneath the target material. To cause some of the atoms or compound species from the piece of target material to deposit themselves on the substrate surface, an electrical DC, pulsed DC, AC or RF power supply is connected between the target material (typically via a bonded backing plate) and the chamber, and a low-pressure (e.g., 0.5 mTorr to 30 mTorr) inert gas (e.g., Ar) and/or a reactive gas (e.g., N2, O2 or the like) medium is established within the chamber. When a high enough voltage is applied to the target material, a plasma is formed with the atoms being released by the target material via ion bombardment by the plasma ions. The gas in the chamber may have a low pressure so that a majority of the atoms from the target material travel from the target material onto the substrate surface without colliding with any gas molecules (i.e., negligible or minimal scattering of the sputter species). In many thin-film deposition applications, to obtain the best quality thin films, it is desirable for the atoms to strike the surface of the substrate at a 90xc2x0 angle (i.e., normal incidence) which provides a collimated stream of atoms.
In order to improve the quality of the thin film produced by the sputtering apparatus, the target may be located a greater distance from the substrate, which is known as a long-throw or natural sputtering apparatus. In the long-throw or natural sputtering apparatus, the atoms from the target material travel a longer distance so that the atoms that are not going to strike approximately perpendicular to the surface of the substrate, and within a space cone with relatively narrow angular distribution around the central normal axis, may strike the sides of the sputtering chamber. Thus, a larger percentage of the atoms or sputter species from the target material strike the surface of the substrate close to a perpendicular angle within a narrow angular distribution from the normal axis. The long-throw sputtering apparatus has several limitations. First, because the atoms or sputter species are traveling a greater distance to the surface of the substrate, they may strike more gas molecules due to scattering collisions and form poorer quality films unless the pressure of the gas in the sputtering chamber is reduced. If the gas pressure is reduced too much, however, there will not be sufficient gas pressure in the chamber to sustain a stable plasma. Typically, the long-throw sputtering processes require gas pressures preferably below 1 mTorr, thus limiting the process window for this technique. Moreover, the long-throw sputtering systems require more stringent vacuum pumping due to larger process chamber volumes and surface area.
Another technique to improve the quality of the thin films (i.e., improving the total number of atoms or sputter species which strike the surface of the substrate at a perpendicular angle or near perpendicular angles) is to place a perforated plate or a physical collimator in the chamber between the substrate and the target surface. This apparatus is called a physical collimation sputtering apparatus. The perforated plate has a predetermined aspect ratio (i.e., the ratio of the height of the hexagonal or circular holes in the plate to the diameter of the holes in the plate) so that most atoms which pass through the plate will strike the surface at approximately a right angle or within a narrow-angle cone around the perpendicular axis. Thus, the perforated plate acts as a spatial filter for the atoms or sputter species and prevents the atoms or sputter species emitted from the target material at more than some predetermined angle (outside a predetermined cone) from striking the surface of the substrate. The atoms or sputter species which strike the plate, but do not pass through the physical collimator holes, are deposited on the plate or within the holes. Therefore, as the plate is bombarded by more and more atoms, the holes of the plate will gradually become coated and eventually plugged up and the plate must be replaced after processing a certain number of substrates. Therefore, the total lifetime of the plate in the chamber is limited and the plate must be replaced often which is time consuming and reduces the overall equipment uptime. With either approach, the maximum collimation that can be achieved is limited.
Therefore, it is desirable to provide a sputtering apparatus which provides a more collimated stream of atoms or sputtering species which avoids these and other problems of known devices, and it is to this end that the present invention is directed.
In accordance with the invention, a dual collimated sputtering apparatus and method are provided which improves step coverage and bottom coverage in large aspect ratio contacts and vias in a semiconductor integrated circuit chip, improves bottom coverage and step coverage for barriers, liners in vias, or trenches in a semiconductor device, and reduces encroachment in a lift-off patterning structure for an abutted junction (the latter used in magnetic thin-film heads). The dual collimator in accordance with the invention may have a long-throw collimator combined with one or more physical collimators. The long-throw collimator provides some initial collimation and the subsequent one or more physical collimators provide additional filtering of the sputtered flux which enhances the overall degree of collimation. In addition, since the long-throw collimator already ensures that some non-collimated atoms or sputtered species strike the walls (or shield walls) of the deposition chamber, the perforated plates of the physical collimator block a smaller fraction of the incoming flux (i.e., atoms) so that the overall lifetime of the perforated physical collimator is increased. The dual collimator apparatus may also be operated at low pressures (e.g., such as less than 2 mTorr) so that the probability of atom scattering due to a collision with background gas atoms (within the long-throw collimator) is minimized so that more collimated atoms or sputtered species strike the surface of the substrate.
In addition, the bottom coverage for a 3:1 aspect ratio (AR) semiconductor via hole improves from about 30% using conventional physical collimation to as much as 50% using the dual collimation apparatus in accordance with the invention while the sidewall coverage of the via hole is not significantly affected (resulting in continuous coverage of topological features). As compared to a sputtering process with no collimation, dual collimation in accordance with the invention also reduces encroachment for lift-off structures by a factor of three. The sidewall angle of a metal layer deposited on a lift-off structure with respect to the horizontal plane increases from 100 for no collimation and 160 for long-throw collimation to 320 for dual collimation in accordance with the invention (the exact sidewall angle can be increased or decreased by changing the dual-collimation parameters).
In addition, the full width half maximum value (FWHM) of the sputtered flux incident on the wafer decreases from xc2x1550 for no collimation and xc2x145xc2x0 for long-throw collimation to xc2x125xc2x0 for dual collimation indicating that more of the flux with the dual collimator strikes the wafer at close to a perpendicular angle. The dual collimation process in accordance with the invention also provides a well defined abutted junction in which the permanent magnet contacts of, for example, a magneto-resistive (MR) sensor may be placed with minimal overlap.
For a chromium (Cr) deposition, the deposition rate decreases from 100 xc3x85/kW/min with a conventional long-throw collimator to only approximately 28 xc3x85kW/min with the dual collimator in accordance with the invention. This decrease in the rate of deposition may be attributed to the increased degree of collimation of the deposited atoms. In addition, a Cr (30 xc3x85)/Co82Cr6Pt12 (350 xc3x85)/Cr(1150 xc3x85) stack processed using the dual collimation process in accordance with the invention shows excellent magnetic properties with a coercivity of 1630 Oe and a squareness of 0.91.
To further improve the dual collimation process which increases the deposition uniformity and increases the mean number of wafers that may be processed between cleans (i.e., an indication of how quickly the perforated physical collimator plate becomes blocked), a novel perforated plate of a physical collimator and a new collimator shield are disclosed.
In accordance with the invention, a physical-vapor deposition apparatus for depositing material from a target onto a substrate is provided comprising a deposition chamber, a target holder housed within the chamber holding a target from which a deposition flux is generated, a sputtering energy source such as a DC magnetron source, a substrate holder housed within the chamber beneath the target holder, and a physical collimator between the target holder and the substrate holder for controlling an amount of deposition flux impinging on the substrate. The apparatus may further comprise a second collimator to provide, in combination with the physical collimator, a collimated deposition of material onto the substrate.